Reimplement file system layer

1. Added a dedicated StylesDirWatcher object, which makes the StyleEngine care about what it should.  Any file watcher object would do the job here, I guess but since there's nothing available in QML natively, let's do this. Since this is a breaking change made the watcher available from version 1.1 and reimplemented the old API in terms of it (marked it as deprecated).
2. Reimplement the style sheet and default style sheet APIs in terms of QUrl.  For now only localFileUrls are supported still, but this should be simple to change later to support qrcs or QNetworkAccessManagers.  Since the two urls are independent now default and user style sheet can be in two completely different places.
3. Explicitly watch the set style sheet and default style sheet instead of watching the containing folder should fix the watching issue on windows.
4. Updated the documentation and fixed some documentation issues.